Measuring wavefront tilt using shearing interferometry

Abstract

Shearing interferometry is a well-established technique for high accuracy optical testing. During evaluation usually piston and tilt in the wavefront are neglected because the interest is in higher order surface or wavefront aberrations. Looking for absolute testing of elements or systems and similar tasks, the evaluation of the tilt in the wavefront between measurements is important too. Several types of shearing ineterferometers are in use. The paper discusses briefly tilt measurement in rotational- and radial- shearing interferometers, but further details lateral shearing interferometers. In lateral shearing interferometry only a difference of the wavefront sheared with itself is measured and therefore wavefront tilt does not show up as fringes, only as a bias to the fringe position. The problems associated with measuring tilt accurately using the standard lateral shearing configuration are discussed and a technique using a variable shear, which allows making wavefront tilt visible to the operator in form of fringes is described. Several solutions to implement this variable shear approach are presented. In all types of shearing interferometer a close look has to be kept at the spatial coherence of the wavefront under test. In general the spatial coherence has to be large enough yield good fringe contrast for the desired shear. In UV applications Excimer-lasers don't have high spatial coherence and high spatial coherence is not desired anyway to reduce coherent noise in the system. Relating to this we discuss solutions for dealing with low spatial coherent light for the variable shear technique. Measurement examples of tilt using variable shear with lateral shearing interferometry and a comparison to a Twyman-Green interferometer in the UV region are presented as well.